BOROHYDRIDE REDUCTION OF COBALT IONS IN WATER - CHEMISTRY LEADING TO NANOSCALE METAL, BORIDE, OR BORATE PARTICLES

The reduction of Co2+(aq) by NaBH4 has been studied with mechanistic and stoichiometric factors as the focus. The primary product of this reduction in aqueous solution is nanoscale Co2B particles, which are formed possibly through the intermediacy of [(H2O)5CoOH(BH2)HOCo(H2O)5]3+, and this complex ion is reduced by electrons provided by 3 more mol of NaBH4. The ratio of BH4-/Co2+ required is 2, and the overall balanced equation is 2CoCl2 + 4NaBH4 + 9H2O --> Co2B + 4NaCl + 12.5H2 + 3B(OH)3 However, a side reaction where NaBH4 reacts directly with H2O becomes important if sufficient time is available before Co2+ addition or if Co/Co2B particles are allowed to catalyze it. In this way NaBO2 can be formed that can react with Co2+ to give Co(BO2)2 as a product, which precipitates. A secondary product is metallic Co particles, which are formed by heat treatment of Co2B/Co(BO2)2 Mixtures, or by sacrificial oxidation of Co2B. A tertiary product is Co3B, and this is formed by heat treatment of a mixture of Co2B and Co. These studies show that the primary product Co2B is best prepared by rapid mixing of Co2+(aq) with solid NaBH4, and the Co2B precipitate must be handled under anaerobic conditions. However, by adjusting conditions of reaction time, mixing procedure, and concentrations, a mixture of Co2B and Co-(BO2)2 can be produced, which upon heat treatment yields metallic Co nanoscale particles plus boron oxides. Magnetic and physical properties of the fresh Co2B are as follows: surface area = 15.9 M2/g, crystallite size = <2 nm, sigma = 48 emu/g, Hc = 260 Oe. For Co particles, properties are as follows: surface area = 9.1 m2/g, crystallite size = <2 run, sigma = 53 emu/g, Hc = 333 Oe.